A thin film electroluminescent display device typically comprises several thin film layers and is adapted to emit visible light upon the imposition of a voltage across the electrodes of the device. The electrodes typically include front and rear patterned electrodes. The active, light emitting element of the device is a thin film phosphor layer which, by the principle of electroluminescence, is excited to emit light in an electric field. In addition to the phosphor layer there are the aforementioned electrodes, one or more dielectric layers for electrical isolation and, if desired, a dark field for contrast enhancement. In addition, there is commonly provided a substrate upon which the layers are formed and some type of means for encapsulating and sealing the device.
The phosphor layer generally comprises a host substance doped with an activator. A host substance that is commonly used is zinc sulfide. The dopant or activator is manganese. Such a phosphor layer emits in the yellow portion of the spectrum at about 590 nm, and has excellent color stability. Another common host substance is zinc selenide.
To provide a properly operating device the phosphor layer must be free from impurities and, moreover, must be uniform as to composition, thickness and doping level. Furthermore, it is essential that the correct stoichiometry and crystal structure be maintained and that the dopant atoms enter the crystal lattice as substitutes for zinc atoms, rather than interstitially.
In presently used processes the zinc sulfide and manganese are co-deposited employing thermal and electron beam deposition techniques. One technique employs a single pumpdown system for thermal evaporation of zinc sulfide along with simultaneous electron beam evaporation of manganese metal. In such co-evaporation systems, rate control is difficult, especially with respect to the dopant, as the rate monitor is generally swamped by the disproportionate high percentage, of about 99%, of zinc sulfide. Furthermore, uniformity over a large area, required for high volume production, is virtually impossible because the evaporation sources are small in comparison with the substrate area. Reproducibility is also a notorious problem with these co-evaporation systems.